Communication satellites typically process signals received and transmitted in a number of communication channels. To separate and combine the communication channels, the satellite may make use of a number of filters. Filters involved in signal processing are designed to meet often strict requirements on signal quality. Gain and group delay variation as a function of frequency can cause signal degradation. It is therefore desirable that filters involved in signal processing exhibit close to gain flatness and group delay flatness where possible.
In non-minimum phase filters, cross-couplings within the filters have been used to equalise the group delay of the filters. This technique requires a higher order filter and there is a limit to the percentage of the bandwidth of the filter which can be corrected.
It is also known to use external networks to equalise group delay across the passband of a filter. For example, external one-port networks have been used to add appropriate delays to signals in the passband.
Moreover, it is known that the quality factor (Q) of the resonators of a filter can be adjusted in order to give some flattening of the passband. The Q of a resonator is a measure of the strength of the damping of its oscillations. To obtain a flat passband, very high Q filter resonators are conventionally used, which results in a filter with a relatively large size. To obtain a flat passband with lower Q resonators, pre-distortion, the introduction of complex transmission zeros or lossy cross couplings in the filter have been suggested. The use of complex zeros or lossy cross couplings to flatten the passband means that an increase in the complexity and the order of the filter is required.
The invention aims to improve on the prior art.